- Fixed issues with field objects which do not inherit the attribute 'validators' from failing.
- Fixed issues with field objects which do not inherit the attribute
'validators' from failing.

AttributeError: 'ObjectIdField' object has no attribute 'validators'.
losintikfos/flask-mongoengine,gerasim13/flask-mongoengine-1,losintikfos/flask-mongoengine,gerasim13/flask-mongoengine-1